Most color picture tubes presently being manufactured are of the line screen slit mask type. These tubes have contoured rectangular faceplates with line screens of cathodoluminescent materials thereon and somewhat similarly contoured slit-apertured shadow masks adjacent to the screens. The mask slits are aligned in vertical columns, with each column containing a plurality of slits that are vertically separated by bridge or web portions of the mask.
Such line screen slit mask tubes are screened by a photographic method that utilizes a line light source, such as disclosed in U.S. Pat. No. 4,049,451, issued to H. B. Law on Sep. 20, 1977. The use of a line light source to form continuous phosphor lines, however, has an inherent geometric problem that must be solved. Because of the substantial curvatures of the shadow mask and tube faceplate, the images of the line light source that pass through the apertures off the major and minor axes of the mask are angled or skewed relative to the intended straight lines. If uncorrected, such skewing of the line light source images results in the formation of phosphor lines that are relatively ragged.
There have been several techniques suggested for solving the light source image skew problem. One solution is disclosed in U.S. Pat. No. 4,516,841, issued to Ragland on May 14, 1985. That patent teaches the use of a cylindrical-shaped lens located near a line light source during exposure of photosensitive material on the faceplate. The longitudinal axis of the cylindrical lens is oriented perpendicular to the longitudinal axis of the line light source. Because of the presence of the lens, the images of the line light source, projected through the slits of the mask onto the photosensitive material at locations off the major and minor axes of the panel, are rotated toward parallelism with the minor axis, thereby resulting in exposure of smoother lines on the photosensitive material.
In a modern color picture tube, the screen edges are perfectly rectangular and the phosphor lines are essentially vertical, depending on mask and panel contours. The cylindrical lens now in use to correct light source image skew has a constant radius across its width, producing an increasing skew correction for increases in distance from the major axis of the lens, which is parallel to the central longitudinal axis of the lens cylindrical shape. Since the skew angle of the line light source image and the skew correction angle provided by the lens vary by different amounts, the skew correction of the lens must be compromised by substantially balancing overcorrection in one area of the screen with undercorrection in another area of the screen. This compromise correction can produce a loss of color purity tolerance in a finished tube, because it results in the width of a phosphor line not being constant over the screen due to the remaining skew. Thus, in one example of a 27 V tube using a cylindrical skew correction lens with a 3.9 inch radius, a maximum skew angle of plus 3.5 degrees was noted at the top of the screen, between the minor axis and the corner, and a skew angle of minus 0.9 degree was noted at the corner. The skew angle of 3.5 degrees causes formation of wider phosphor lines, which results in a loss of tolerance of about 35 micrometers. Furthermore, a large skew angle also creates some amount of line necking which may be visible and thus objectionable in a finished tube. Therefore, there is a need to improve the design of skew correction lenses to reduce the amount of skew angle remaining during screening. The present invention meets this need.